Steam turbine installation



Nova & 396G P. .1. Ric/Ann STEAM TURBINE INSTALLATION Filed June 21,1957 INVENTO}? Fig.3 16 re 15 14 MMMMWL PIERRE JEAN 121cm]:

ATTORNEYS STEAM TURBINE INSTALLATION Pierre Jean Ricard, Paris, France,assignor of one-half to Societe des Forges et Ateliers du 'Creusot,Paris, France, a company of France Filed June 21, 1957, Ser. No. 667,125

Claims priority, application France June 30, 1956 4 Claims. (Cl. 60-67)In steam turbine installations, the condensed water is usually reheatedin a series of water reheaters consisting of tubular heat exchangers fedby steam drawn off from the turbine. The heat given up in such areheater by the steam drawn 01?, between the temperature at which itleaves the turbine and a temperature equal to or slightly higher than,the saturation temperature corresponding to its pressure, that is to saythe reheating heat of the reheater, is given up to the water in aparticular zone known as the de-superheating zone, which precedes, inthe direction of flow of the steam, the so-called saturated zone, wherethe steam gives up its latent heat of vaporisation.

In such an installation, the heat exchange in the desuperheating zoneintroduces an element of irreversibility into the thermodynamic cyclebecause the temperature of the steam may be much higher than that of theWater,

especially in installations comprising one or more resuperheatingoperations.

The present invention has the purpose of increasing the efliciency ofthe installation by reducing the degree of irreversibility in thethermodynamic cycle.

According to the invention, the installation is characterised by thefact that it comprises means for sampling a fractional delivery of waterin at least one condensed water reheater at the point where the waterhas finished reheating in the saturated steam zone of the reheater, aheat-exchanger being provided in order to reheat the said fractionaldelivery of water to a temperature slightly lower than that of the steamdrawn off from the turbine,

and means being finally provided to re-introduce the said fractionaldelivery into the main water circuit, the point of re-introduction beingsituated after the first water reheater in the direction of flow,reckoning from the output of the reheater set, whereof the outputtemperature is lower than the temperature of the fractional delivery ofreheated water.

The delivery of water drawn oif can, in certain cases, be divided intotwo parts after a predetermined degree of reheating, one of the saidparts continuing alone to be reheated, the two parts being re-introducedinto the main water circuit at two separate points suitably chosen as afunction of the respective final temperatures of the two parts.

An installation according to the invention is diagrammaticallyillustrated in the attached drawing, and described hereinafter.

In the drawing:

Figure l is an overall diagram of the installation.

Figure 2 is a diagram of the temperatures in an installation as shown inFigure 1.

Figure 3 is a partial diagram showing a subdivided water take-0E.

Figure 4 is a diagram of the temperatures in an installation wherein thewater drawn ofi is partially vaporised.

Figures 5 and 6 show two types of water reheaters incorporating theheat-exchangers used.

2,959,013 Patented Nov. 8, 1960 Figure 7 shows a variant of anembodiment of the installation in the case where the water reheater isof the mixer type.

Figure 8 is a modification of Figure 1.

The diagram in Figure 1 shows an initial water sampling system in thecase of a re-superheating turbine, for which only three steam take-offsare illustrated.

In this diagram, 1 designates the high-pressure part of the turbine, 2the re-superheater, 3 the lower pressure turbine, and 4 the condenser.Three condensed water reheaters 5, 6 and 7 are fed by the three steamtake-offs 8, 9 and 10 respectively. The saturated zones of thesereheaters are designated by 5a, 6a and 7a respectively, while thede-superheating zones are designated by 5b, 6b and 7b. The condensedwater outlets of the reheaters are illustrated at 8a, 9a and 10a.

The reheater 5 is fed by water from condenser 4 through conduit 14 toreheater 5a comprises a water take-01f 11 which is reheated by thetake-off 8 in the zone 5b, the heat-exchanger used replacing, in thiszone, the usual de-superheating exchanger for the steam drawn off.

The delivery of reheated water 11 is conveyed at 12 to the main waterdelivery 14c at the outlet of the last reheater if its temperature ishigher than that of the water at the outlet of the reheating set of theturbine.

In the opposite case, the delivery of reheated water 11 is conveyed at13 in Figure 8 to the condensed water circuit 14b after the firstreheater, reckoning from the outlet of the reheating set, for example 6,whereof the outlet temperature is lower than its own temperature. Waterfrom reheater 5a is taken by conduit 14a to reheater 6a, then throughheater 6b and, by conduit 14b, to heaters 7a and 7b to water deliveryconduit 140.

The water take-oils are contrived in the same fashion on each of theother reheaters; they are not illustrated for the sake of clarity in thediagram.

In the diagram in Figure 2, wherein the quantities of heat exchanged areplotted as abscissae, and the temperatures as ordinates, the curves Tand 2 respectively represent the variation of steam and watertemperature as a function of the quantity of heat exchanged in unittime. The two curves T and t are of the shape shown because the specificheat of water is an increasing function of temperature, and the specificheat of steam on the other hand is a decreasing function of temperature.

The point A represents the temperature of the steam and the point G thetemperature of the water at the end of the de-superheating zone. Thehorizontal line .98 represents the saturation temperature correspondingto the steam pressure in the saturated zone, and the curve gG representsthe end of water temperature variation in the said zone. The point A maypossibly coincide with the point S, or be situated slightly above thepoint S, according to the surface area of the de-superheating exchanger.The interval 86 is generally of the order of 2 to 3 C. in an ordinaryreheater.

The improvement obtained in the efiiciency of the installation becomesgreater as the two curves T and t approach one another, and consequentlyas the delivery of water drawn off becomes smaller, but the saiddelivery must be fixed by a diaphragm or valve (not illustrated), insuch a manner that:

(a) there is always sufficient difference in temperature between thesteam and the water. Since the interval AG is small, this condition isrealised if the slope of the curve t is not greater than that of'thecurve T at the points G and A respectively.

(b) the difference in temperature at the end of the heat-exchanger wherethe fluids are hottest, represented by BC, remains sufliciently largefor the area of the exchanger not to make this appliance too costly.

In the cast in which the condition (a) is determinative, the efficiencyof the cycle may be increased by bypassing from the delivery ofwaterdrawn off, during reheating, a partial delivery which continues to bereheated alone in accordance with the curve D; theffresultbfthis is areduction in the final temperature diifereiice,' which becomes BEinstead of BC. v

This dividedtake-off, which forms a "sdpple'rrientary feature of theinvention, is illustrated in thedia'gram in Figure 3.

In this diagram, 8 is the steam take-01f, 14 is'the 'r'nain waterdelivery,15 is the delivery'of water driawn'joif, which divides, afterhaving been Subjected to" a' first reheating operation, into a'delivery11 which isreturned to the circuitat a'pointchos'enin accordance'withthe foregoing, for example before one of' the subsequent reheaters, anda delivery 16 'which continues to be reheated, and is then returned tothe circuit at another suitably chosen point,'for example at the outletof the last reheater.

The efiiciency of the cycle could be further increased by lay-passing,from the delivery of water drawn off, a second amount, which wouldbe'r'eheated in accordance with the curved in the diagram in Figure 2,the final diflerence in temperature then'being represented by BF, and soforth.

Thewater take-offs do not require any pump other than the pumps normallyprovided in a steam turbine installation in the case of water reheatersof the so-called high-pressure type, that is to say situated on thedelivery side of the pump feeding the steam generator. In fact, in thiscase, the water drawn off only has to pass through the exchanger whichreheats it, while the main water delivery has to pass through thesubsequent water reheaters, which are at least one in number. There istherefore enough pressure available to ensure circulation of thedelivery drawn off.

On the other hand, a special pump is required in the case oflow-pressure reheaters situated before the feed pump, if the delivery ofwater drawn off must be conveyed to the high-pressure circuit. However,in order to avoid having to install a low-power high-pressure pump, thedelivery of water drawn off may be conveyed to the body of the reheaterprovided itself, where it mixes with the blow-olfs coming from thehigher-pressure reheaters, instead of being conveyed to the main waterdelivery before it enters the reheater. The two processes are in factsubstantially equivalent as regards efiiciency.

In certain cases, the temperature at which the steam is drawn off may besufficiently high for the delivery of water drawn off to be partiallyvaporised. The temperatures then vary as shown in the diagram in Figure4, wherein the curve T represents steam temperature, and the curve 1represents water temperature, this latter curve having a horizontalportion HI during which the water becomes partially vaporised. Theneither the delivery of water drawn off may be increased in order toprevent vaporisation, the temperature then varying in accordance withGK, or partial vaporisation in accordance with HI may be accepted, whichincreases the efliciency in so far as the condition (a) defined abovedoes not cause any limitation in this method. The heat-exchanger thencomprises a zone where partial vaporisation occurs, and the tubes mustbe so shaped in this zone that the steam cannot accumulate at a bend andhinder circulation.

his then advantageous to convey the mixture of water and steam leavingthe exchanger either to the container of the steam generator if thetemperature of the'said mixture is higher than that of the saturatedsteam in the last reheater, or, in the opposite case, to the saturatedzone of the first reheater for which the steam temperature is higherthan that of the mixture.

All kinds of construction of Water reheaters can be used for putting theinvention into practice.

Figures 5 and 6 show, in more or less diagrammatic form, and by way ofexample, two water reheaters with heat exchanger incorporated forreheating the delivery of water drawn off.

The exchanger illustrated in Figure 5 is of the waterbox type, and thatin Figure 6 of the header type.

In these two figures, 21 and 22 respectively designate the saturatedzone and the heat-exchanger incorporated in order to reheat the Waterdrawn off, while 23 and 24 respectively designate the steam inlet andthe condensed water outlet.

In the reheater shown in Figure 5, the water enters at 25, the mainwater delivery outlet is situated at 26, and th outlet for the waterdrawn off at 27.

The reheater shown in Figure 6 comprises a water inlet header 28, anintermediate header 29 and an outlet header 30 for the water drawn off.

If the water reheater is of thernixer typ'e,'which is so in the case ofa degasifier, the delivery of'water drawn off can be reheated in aspecial exchanger. The diagram of such an exchanger is shown in Figure7, wherein 8 is the steam take-off, 14'the main water delivery, ll thedelivery of water drawn 011, 19 the pump'which'a'spirates the waterleaving the reheater," and '20 the special exchanger.

What is'claimed is:

1. In a steam' turbine installation including a turbine and a turbinecondenser, a plurality of feedwater heaters, a first steam zone and asecond steam zone for each of said heaters, a steam path from theturbine to said first zone and then to said second Zone for each of saidheaters, a feedwater path from the turbine condenser to said second zoneof the first of said heaters, a tap in said first heater between saidzones of said first heater dividing the reheated feedwater when thefeedwater has been heated in said second steam zone of said firstheater, a part of the feedwater passing through said first steam zone ofsaid first heater for heating to a temperature less than butapproximating that of the steam' from the turbine, the remainder of thefeedwater passing from said tap to said second steam zone of an adjacentone of said heaters and then passing in series through said second andthen said first zones of the remainder of said heaters and said part ofthe feedwater after heating in said first zone of said first heaterentering the feedwater path downstream of the adjacent one of saidfeedwater heaters to said first feedwater heater.

2. In an installation as described in claim 1, a tap in said first steamzone of said first heater dividing the feedwater heated in said firststeam zone into two parts one 'of which is further heated in said firststeam zone, said two feedwater parts being reintroduced into thefeedwater path downstream of the adjacent one of said feed water heatersto said first feedwater heater.

3. In an installation as described in claim Zysaid further heated partsof said two parts of said feedwater being introduced into the feedwaterpath at a point after the last of said heaters in the directionof'feedwater flow.

4. An installation as described in claim 1 in which each of said heatersis a tubular heat exchanger having a first steam zone and a second steamzone.

References Cited in the file of this patent UNITED STATES PATENTS GreatBritain Apr. 6, 1955

